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SUSPENDED SOLIDS AND WATER QUALITY
Total Suspended Solids (TSS) is comprised of organic and mineral particles that are transported in the water column. TSS is closely linked to land erosion and to erosion of river channels. TSS can be extremely variable, ranging from less than 5 mg L-1 to extremes of 30,000 mg L-1 in some rivers. TSS is not only an important measure of erosion in river basins, it is also closely linked to the transport through river systems of nutrients (especially phosphorus), metals, and a wide range of industrial and agricultural chemicals.In most rivers TSS is primarily composed of small mineral particles. TSS is often referred to as 'turbidity' and is frequently poorly measured. Higher TSS (>1000 mg L-1 may greatly affect water use by limiting light penetration and can limit reservoir life through sedimentation of suspended matter. TSS-levels and fluctuations influence aquatic life, from phytoplankton to fish. TSS, especially when the individual particles are small (< 63µm), carry many substances that are harmful or toxic. As a result, suspended particles are often the primary carrier of these pollutants to lakes and to coastal zones of oceans where they settle. In rivers, lakes and coastal zones these fine particles are a food source for filter feeders which are part of the food chain, leading to biomagnification of chemical pollutants in fish and, ultimately, in man. In deep lakes, however, deposition of fine particles effectively removes pollutants from the overlying water by burying them in the bottom sediments of the lake. In river basins where erosion is a serious problem, suspended solids can blanket the river bed, thereby destroying fish habitat.
Sediment yield, expressed as tonnes km-2 year-1 is calculated by dividing the total annual TSS load (tonnes) by the surface area of the watershed (km). Sediment yield is a key indicator of land erosion. Estimates of the average global annual sediment load to the world oceans varies from 15 to 30 billion tonnes (Ref. 5,6,7).
The large sediment loads to oceans in Southeast Asia are two thirds of the world's total sediment transport to oceans. This arises from the combination of active tectonics, heavy rainfall, substantial local relief with steep slopes, and erodible soils including the loess belt of northern China. The Huang He (Yellow) produces 1080 million tons of sediment annually; 480 for the Chang Jiang (Yangtze), 460 for the Ganges, and 710 for the Brahmaputra. Reservoir construction (Indus, and a future dam on the Chang Jiang) may affect these numbers in the future. In comparison, low relief, low precipitation and permafrost greatly reduce turbidity and sediment loads in Siberian rivers (about 15 million tonnes for each of the Ob, Yenissei and Lena) (Ref. 7).
Time series of instantaneous TSS loads (kg s-1) provides useful information about the physical behaviour of rivers. Because total suspended solids concentration is partly a function of discharge, TSS load increases as discharge increases. In many rivers, the amount of sediment (solids) in transport (the load) can vary over three or more orders of magnitude during the year. The Huang He in China is an example of this relationship.
Typical of most rivers, TSS peak loads in the Rhine River occur only during short periods (days), and often are not identified when rivers are sampled on a bi-monthly or monthly basis. This can lead to major errors in calculating sediment transport.
Total dissolved solids (TDS) concentrations are generally inversely proportional to river discharge (Q). This relationship is the result of the mixing of more mineralized groundwaters that dominate during baseflow, with more dilute surface runoff waters. This inverse relationship can been seen when seasonal TDS variation is plotted with corresponding discharges. Relative variations can be quite high: from 30% for rivers with low TDS levels (Sagami, Tennessee) to 200% or even 400% for more saline rivers (Ebro, Murray). TDS can rise proportionately with flow (Q) in arid regions where leaching of salt deposits can occur during the rising stage of the flood.
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